Electrical cables which are used for supplying electrical energy to submersible oil well pumps are typically of flattened, cross-sectional shape and must be able to survive and perform satisfactorily under extremely adverse conditions of mechanical stress.
Such cables are subjected to mechanical stresses in several ways. It is common practice to fixedly attach the cables to the housing of the electrical submersible pump or oil well tubing by means of bands or straps which may crush the cables and thereby seriously degrade the quality of the insulation and the electrical integrity of the cable. The cables are also subject to impact damage during installation and high compression loads during and after installation. To resist these forces, it is conventional to apply an outer metal armor to the cable and to enclose the individual conductors with layers of materials chosen to enhance their strength characteristics. Such measures are sometimes not adequate to provide the necessary insulation protection, especially against edge impacts and edge abrasion.
An additional problem arises as a result of down-hole pressures, which can be in the hundreds or thousands of pounds per square inch, to which the cables are subjected. Typically, the insulation surrounding the conductors in a cable contains micropores into which gas is forced at these high pressures over a period of time. Then, when the cable is rather quickly extracted from the well, or the pressures applied to the cable by the column of fluid in the well bore change dramatically due to intermittent operation of the pump powered by the cable, there is insufficient time for the intrapore pressure to bleed off. As a result of repeated decompressions, the insulation tends to expand and contract and can rupture rendering the cable weakened or useless.
In U.S. Pat. No. 4,409,431, assigned to the same assignee as the instant invention, there is described a cable structure which is particularly suitable for use in such extremely adverse environments. The structure protects the cable against inwardly-directed compressive forces and provides for the dissipation of heat from the cable interior to the surrounding environment which is an important feature in high temperature operating environments, for reasons discussed therein, as well as resistance to decompression expansion of the insulation.
As described in said U.S. Pat. No. 4,409,431, the cable protective structure includes one or more elongated, force-resisting members which extend parallel and adjacent an insulated conductor of the cable. The members are rigid in cross-section to resist compressive forces which would otherwise be borne by the cable conductors. For applications requiring the cable to undergo long-radius bends in service, the elongated support may be formed with a row of spaced-apart slots which extend perpendicularly from the one edge of the member into its body to reduce the cross-sectional rigidity of the member in the slotted areas so as to provide flexibility in the support to large-radius bending about its longitudinal axis.
As described in my copending patent application Ser. No. 390,308 filed June 21, 1982 issued as U.S. Pat. No. 4,454,377 on June 12, 1984 and assigned to the same assignee as the present invention, for certain service applications, it may be preferred that the electrical insulating sheath on the cable conductor not be in direct contact with the slot openings. This is because the slot openings in the support member may allow highly corrosive materials to gain access to the jacket composition by flowing inwardly through the slots. In addition, the sharp corner edges formed by slotting may cut into or abrade the underlying cable jacket upon repeated flexing of the cable.
The cable protective structure of said copending application Ser. No. 390,308 is made of a composite structure which utilizes an elongated force-resisting member of good thermal conductivity positioned adjacent the insulating conductor sheath. This member comprises a channel member of U-cross-sectional shape. A smooth, bendable liner may be mounted within the channel facing the insulation of the adjacent conductor to bridge the slots in the member and thereby protect the underlying insulation from abrasion by the slot edges during bending of the channel member.
The exterior jacket or armor, the liners and the channel members all serve to protect the conductor insulation, and hence the cable, from damage caused by compression forces, impacts and decompression expansion.
Supplementary resistance to compressive forces may be obtained with a cable constructed in accordance with my copending applications Ser. Nos. 429,530 and 429,781, filed on Sept. 30, 1982 issued as U.S. Pat. Nos. 4,453,035 and 4,453,036, respectively, on June 5, 1984, and assigned to the same assignee as the instant invention.
For certain service applications and particularly oil well applications, the cable must be able to be axially inserted and withdrawn through an open space formed between the interior circular wall of the well casing and the exterior surface of the oil well tubing, electric submersible pump housing or other structure to which the cable is affixed. Typically, the cable is mounted on the exterior surface of a centrifugal pump and hence, extends outwardly of the pump housing thereby posing a potential obstruction to a proper fit in the oil well casing. Furthermore, it follows that the thicker the cable in cross-section, the smaller the cross-sectional dimension that the pump must have for both to fit into an oil casing of a given cross-sectional size. Electrically-powered centrifugal pumps, however, are typically much more efficient in large diameters, and thus, it is preferred that the cross-sectional thickness of the associated cable be made as small as possible so that the user can employ the most efficient pump. Since these structures are typically cylindrical, the open space between them is essentially annular in cross-section, being defined by two essentially circular surfaces of different radii.
As mentioned above, for these applications, the cable is subject to very high temperatures and pressures, severe compressive forces in the well and impacts during installation from, for example, hammers or other tools. Hence, it is desirable to use the cables disclosed in my aforementioned patent applications and yet, it is also desirable to minimize the effective thickness of the cable and thus, the possibility that the cable will jam or lodge against the well casing during the insertion or withdrawal of the equipment on which the cable is mounted.
As disclosed in my above-identified copending patent application Ser. No. 447,969, armored cable for oil well applications may have an arcuate cross-sectional shape which conforms to the curvature of the surface on which it is mounted. This reduces the effective thickness of the cable by conforming to and taking maximum advantage of the annular space available between the wall of the well casing and the outer cylindrical surface of the underlying pump housing or tubing. While this arcuate construction reduces the profile of the cable edges to abrasion and edge impacts under extreme usage conditions, the outside edges of the cable and hence, the insulation on the outside conductors may nonetheless receive destructive edge impacts as well as structural degradation resulting from abrading against the wall of the well casing.
My copending patent application Ser. No. 484,977 filed Apr. 14, 1983, issued as U.S. Pat. No. 4,490,577 on Dec. 25, 1984, and assigned to the same assignee as the instant invention, discloses an armored electrical cable structure of arcuate cross-sectional shape which is especially constructed to resist destruction and degradation of the conductor insulation on the outer conductors resulting from impacts and edge wear caused by abrasion. The structure also provides the additional benefits of resisting decompression expansion of the conductor insulation, attack by corrosive agents as well as the conduction of heat from the interior parts of the cable structure to the cable exterior for dissipation.
As described in said application Ser. No. 447,969, the arcuate cross-section results in manufacture of the cable by drawing the initially completely flat cable structure through coacting forming rollers which bend the armor and the interior components of the cable, including the conductors and the force-resisting members into an arcuate cross-section. As illustrated therein, the outer side portion of the cable has a greater radius of curvature and hence, a longer circumference in transverse cross-section, than the inner side portion. To compensate for this differential in transverse circumferential widths between the opposite arcuate side portions of the cable, the width of the upper surface 26 of the force-resisting member opposite and underlying the outer side portion of the jacket 11 is made wider than the lower surface 27 of the member opposite and overlying the inner side portion of the jacket 11 with its narrower circumferential width.
In accordance with this invention, an arcuate cable includes insulated conductors and force-resisting members adjacent thereto. The force-resisting members are of different widths in transverse cross-section with the greater width adjacent the circumferential longer side portion of the cable to provide the desired enclosure to the outermost insulation layer on each insulated conductor.